687. 用經緯儀正鏡觀測某點天頂距讀數為88°34'40",倒鏡讀數為271°27'40",則其垂直角為?(A)仰角 1°33'30"(B)俯角 1°33'30"(C)仰角 1°26'30"(D)俯角 1°26'30''